"We have reports that they may have in fact crossed borders," House Intelligence Committee Chairman Mike Rogers, R-Mich., told USA Today.
The U.S. State Department said in June it hired mine-clearing teams to find the missiles, known as man-portable air-defense systems, or MANPADS, which Libyan leader Moammar Gadhafi's military stockpiled before the popular uprising that ousted him last month.
The military had amassed nearly 20,000 of the weapons, but some were looted during the fighting and an unknown number went astray.
"There are some worrying indicators that some MANPADS, type non-specific, have left the country," U.S. Army Gen. Carter Ham, who commanded the initial Western military intervention in Libya and heads the U.S. Africa Command, told the newspaper.
Al-Qaida would like to get its hands on the MANPADS, which can fit in a car trunk, Rogers said.
The militant Islamist organization is believed to have produced an hour-long training video on how to use them, CNN reported.
Most of Libya's stockpiles are Soviet Union-era SA-7 Grail MANPADS, low-altitude surface-to-air missile systems with high-explosive warheads and passive infrared homing guidance, U.S. officials said.
Missiles like those have been used in attacks on 40 aircraft, causing 28 crashes and more than 800 deaths since 1975, USA Today said.
They have also been used, along with other MANPAD systems, by insurgent groups in the Iraq and Afghan wars.
The State Department is "very concerned" about the MANPAD smuggling possibility, Assistant Secretary of State for Political-Military Affairs Andrew Shapiro told USA Today.
But "thousands of MANPADS" were likely destroyed during NATO operations "because weapons bunkers were a major target," he said.
Many of the missiles, also known as 9K32 Strela-2s, are also controlled by rebel forces loyal to Libya's transitional government and others are too old to work, the State Department said.
The rebel forces were poised to take control of Gadhafi's birthplace city of Sirte Monday, witnesses said after the forces captured a series of loyalist holdouts in a push through the southern coastal city halfway between Tripoli and Benghazi.
A final push into the rest of Sirte was delayed to let civilians leave, the forces told Britain's Daily Telegraph.
The rebel forces also drove out high-level Gadhafi-regime figures from the airport of northwestern city of Bani Walid, the transitional government said Sunday, leaving loyalists cornered in an area of the city smaller than a square mile near the sea.
Columns of troops backed by tanks were seen moving into the city center on Monday.
National Transitional Council Chairman Mustafa Abdul Jalil, head of the caretaker government controlling much of Libya, said both cities would likely be liberated this week
The NTC said once Sirte falls, they would declare national liberation, the BBC reported.